In the prior art, such implant systems which function as spacers between adjacent vertebral bodies to be fused and replace injured or degenerated intervertebral discs are known:
U.S. Pat. No. 7,077,864 describes an example of a vertebral interbody cage that can be implanted from an anterior, posterior, anteriolateral or lateral position. A cage is an example of a vertebral interbody spacer and spine stabilizer. The cage is filled with bone graft or bone growth promoting material, which promotes the fusion of the vertebrae for long term stability. Advantageously, three screws are used for fixation of the cage, wherein one screw projects at one angle up or down and the other two screws are angled so as to splay in opposite directions. Preferably, the screws are to be inserted through the anterior wall of the cage and through the endplates of hard cortical bone into the softer, more cancellous portion of the bone of the adjacent upper and lower vertebral body to fix the relative position of the cage and vertebral bodies. Furthermore, precautions are necessary to fix the screws in the anterior wall of the spacer or cage in such a way that the screw heads do not protrude outwards of the anterior wall of the cage and that the screws cannot loosen to avoid damaging the major blood vessels that run along the anterior portion of the spine.
Similarly, U.S. Pat. No. 7,232,464 teaches an intervertebral spacer implant with a three-dimensional structure with several boreholes designed to receive screws or other elongate affixation means which can be rigidly connected to the intervertebral implant and are anchored in the adjacent vertebral bodies through penetration of either the inferior or the superior or both of the endplates. The affixation means are typically guided at an angle deviating more than 25°, preferably 35°-55° from the median plane. Such an arrangement of the affixation means ensures anchoring in the compact cortical bone of the endplates of the adjacent vertebral bodies. Again special measures are taken such that the affixation means neither loosen nor protrude, in order to avoid damaging of the major blood vessels.
The fixation of these and other interbody spacers to the vertebral bodies relies on the penetration of the cortical bone of the endplate. Thus the exact placement and angular guiding of the screws is critical. Driving the fixation means through the endplates may weaken the cortical bone of the endplates, compromising the stability of the vertebral bodies. This may be problematic if the bone quality is already weakened by degenerative osteoporosis or traumatic injury or if multiple attempts for the fixation are required during the surgical procedure. Furthermore, during spine surgery access to apply instruments is often limited and it may be difficult to drive affixation means into the vertebral bodies at such pronounced angles required to drive the fixation means from the frontal or a lateral side wall of the intervertebral spacer implant through the endplates of the vertebral bodies.
U.S. Pat. No. 7,255,698 discloses devices for stabilizing vertebral bodies, which devices include an interbody spinal fusion implant and spinal fixation devices secured by a screw to the interbody spinal fusion implant so that loosening of the device is prevented. The spinal fixation devices have a length exceeding the distance between the two adjacent vertebral bodies and are engaged in both vertebral bodies by means of screws or ratchet like structures.
Also in this system, measures have to be taken so that the screw heads do not protrude outwards of the anterior wall of the cage and that the screws cannot loosen to avoid damaging the major blood vessels that run along the anterior portion of the spine. A further potential problem lies in the engagement between the spinal fixation devices and the vertebral bodies. Especially in the case of already weakened bone the fastening primarily relies on a mechanic engagement between a screw or staple with ratcheted structures on the one hand and a relatively thin layer of anterior cortical bone on the other hand. Constant mechanical wear may damage the bone tissue in a vicinity of the screw or staple projection, and this may result in a loosening of the screw or staple.